The present invention relates to improvements in or relating to a secure tag.
Secure tags are used for a number of different purposes. One of the primary uses of a secure tag is to prevent counterfeiting. One type of secure tag that has recently been developed is based on small particles of a rare earth doped host, such as glass. This type of secure tag is described in U.S. patent application No. 2004/0262547, entitled “Security Labelling,” and U.S. patent application No. 2005/0143249, entitled “Security Labels which are Difficult to Counterfeit”, both of which are incorporated herein by reference. These rare earth doped particles (hereinafter “RE particles”) can be applied to valuable items in different ways. For example, the secure tags can be incorporated in fluids which are applied (by printing, spraying, painting, or such like) to valuable items, or incorporated directly into a substrate (paper, rag, plastic, or such like) of the valuable items.
In response to suitable excitation, RE particles produce a luminescence spectrum having narrow peaks because of the atomic (rather than molecular) transitions involved. Known readers for RE particles include (i) a suitable excitation source to stimulate transitions in the secure tag, and (ii) a detector to measure the luminescence emitted in response to the excitation.
The suitable excitation source may be tuned (in the sense that the excitation is selected to optimize luminescence from one or more transitions in the rare earth ions), or high frequency (to stimulate all, or at least many, transitions in the rare earth ions and the host).
Tuned excitation is usually achieved using a narrowband source, such as an LED, in the visible region of the electromagnetic spectrum. High frequency excitation may be achieved using a broadband source having a high intensity contribution from the ultra-violet or low-wavelength visible (for example, between 350 nm and 400 nm) portion of the electromagnetic spectrum. Alternatively, high frequency excitation may be achieved using a narrowband source emitting in the ultra-violet or low-wavelength visible portion (for example, at 395 nm) of the electromagnetic spectrum.
One disadvantage of the RE particles being responsive to excitation from a high frequency source is that low-cost ultra-violet radiation excitation sources may be used for all types of RE particles, rather than having to use a different selective excitation source for each type of RE particle. This may reduce the security of the secure tags.